A water stain is spreading on the ceiling under the AC

What to do right now

1. Turn the cooling off at the thermostat. That stops new water.
2. Put a bucket under any drip and move what you can away from it.
3. If the stain is sagging or the ceiling is bulging, keep people out from under it. Wet drywall can drop.
4. If you can reach the attic safely, look at the pan under the unit with a flashlight. Do not stand on the drywall between the joists.

How the water gets out

The AC's indoor coil runs cold, and moisture from the house air condenses on it and drips into a primary drain pan built into the unit. From there a condensate line, usually white plastic pipe, carries it outside or into a plumbing drain.

Because an attic unit sits above living space, a secondary pan under the whole unit catches overflow, and a float switch in the pan or on the line shuts the AC off when water rises. That shut-off is deliberate: a house that warms up with no obvious fault is often a float switch doing its job.

Why the line clogs

Algae and sludge grow in the line over a humid summer, dust from the coil washes into it, and nests or debris block the outside end. Long cooling seasons give the line more time to clog; here the AC runs from about May to October.

What a technician checks

The primary and secondary drains, whether the secondary pan is rusted through, whether a float switch exists and trips, whether the condensate line has a trap and enough slope, and whether the coil is freezing and thawing. An iced coil sheds more water at once than a line may carry.

What to say when you call

• Where the stain is, and whether it is dripping now
• That the AC is off, and whether it had stopped cooling first
• That the unit is in the attic, and how the attic is reached
• The install year of the system, if known
• Your ZIP code

Common questions

The AC stopped and the thermostat is blank or shows an error. On many systems the float switch cuts power to the thermostat circuit. Clearing the drain, not resetting the breaker, is what brings it back.

Should the ceiling be repaired now? After the leak is fixed and the drywall has dried. The AC repair comes first.